The differentiation of the nervous system depends on a sequence of developmental decisions. These decisions - mediated by cell-cell interactions - include (1) the choice between neuronal versus non-neuronal cell lineages, (2) the subsequent choice among a wide variety of specific neuronal cell fates, (3) the choice of a pathway for axon elaboration, guidance, and targeting, and ultimately (4) the choice of an appropriate target cell with which to synapse. The long-term goal of this project is to understand the molecular basis of each of these developmental decisions, particularly with respect to the specification and organization of neural circuits. In Project 1, led by Martyn Goulding, we will study the circuits in the ventral spinal cord that mediate locomotion. We will focus specifically on transcriptional regulation of the development of V2 and V3 interneurons within these circuits in the mouse, and on the role of the EphA4 receptor in the organizing their connections. In Project 2, led by Sam Pfaff, we will analyze the role of EphA4 signaling in the guidance of motor axons once they exit the ventral spinal cord and navigate into the periphery. We will study how EphA4 may function as both a forward-signaling receptor in motor neurons, and as a reverse-signaling ligand. In Project 3, led by Dennis O'Leary, we will use a series of engineered mouse mutants to study the genetic regulation of the specification of subsets of thalamic neurons and their patterning into distinct nuclei. We will go on to use these findings and mice to study the targeting of thalamocortical axons that arise from these nuclei and their influences on the patterning of the developing neocortex into distinct areas. And in Project 4, led by Greg Lemke, we will study how synaptic connections within topographic connectivity maps are specified. We will exploit a set of mouse mutants in which the expression of the EphA3, EphA5, EphA5, and Eph6 receptors is systematically altered, in order to decipher the role that these receptors play in the topographic wiring of the eye to the midbrain. Together, these experiments will provide us with fundamental new insights into the cellular interactions that mediate neural development, as well as a detailed understanding of their molecular basis.